In the mounting of a semiconductor chip such as an integrated circuit (IC), to a substrate by wire bonding, as the number of electrodes increases, the operating efficiency and reliability are lowered. As a substitute for the wire bonding, a flip chip method has widely been employed because the method requires only a small mounting area, and can render the resultant height smaller and can achieve downsizing of semiconductor packages and improvement of the mounting efficiency as well as speeding up of the operation of electronic apparatus.
As one of bonding techniques by the flip chip method, there is a method in which a semiconductor chip is bonded to a substrate, and then, an insulating resin or a paste containing such a resin (hereinafter, these are collectively referred to as “sealant”) in an uncured state is injected into the gap between the semiconductor chip and the substrate, followed by curing. However, by this method, a period of time for flow the underfill into the void in a length of, for example, 2 cm, requires 6 to 10 minutes. Also, in a large-size chip such that the flow length is more than 2 cm, flow of an underfill is difficult. And yet, the underfill cannot be uniformly flowed, whereby occurrence of voids is remarkable.
Therefore, as an improvement of the above flip chip method, the method shown in FIGS. 2a to 2c is used in which an underfill 2 is dispensed in an uncured state onto the upper surface of a substrate 1 having conductive portions on the upper surface, and a semiconductor chip 3 having bumps (protruding electrodes) 4 on the back surface is mounted on the substrate so that the underfill is spread over the upper surface of the substrate, and then, the underfill 2 is cured by heating from the upper surface of the substrate by means of heating press 5 while pressing the bumps against the conductive portions on the upper surface of the substrate to seal up the gap between the semiconductor chip 3 and the substrate 1. FIG. 2a shows a step of dispensing an uncured sealant, FIG. 2b shows a step of mounting a semiconductor chip, and FIG. 2c shows a step of curing the underfill while pressing the bumps against the substrate, and the method is carried out in this order. For easy understanding of the method, a semiconductor chip having only two bumps is shown in FIG. 2, but, as described below, a number of bumps are generally present per semiconductor chip. Further, there is employed a method in which a substrate having thereon a bump is used, irrespective of whether or not the semiconductor chip has a bump, and the bump is provided at the substrate side and pressed against the bump or electrode on the semiconductor chip side.
The mounting of semiconductor chips by a flip chip method is disclosed in, for example, Japanese Provisional Patent Publications No. 25143/1992, No. 254345/1992 and No. 280443/1992. As materials for the bump, Japanese Provisional Patent Publication No. 25143/1992 describes metals formed by electroplating, Japanese Provisional Patent Publication No. 254345/1992 describes Au, and Japanese Provisional Patent Publication No. 280443/1992 describes various solder alloys, for example, Sn/Pb solder and Pb/In solder.
When mounting is conducted by a flip chip method, even by the above-mentioned improvement method, the gap between the semiconductor chip and the substrate cannot be fully filled with the sealant, and sometimes causing a void. When such a void is present, moisture is condensed in the void in a heat cycle during the use of the resultant package to lower the withstand voltage, causing leakage current. Therefore, establishment of a flip chip mounting method which does not cause void formation is desired.
On the other hand, when wiring is formed on the surface layer of the substrate and/or in the substrate, the upper surface of the resultant substrate has fine unevenness to prevent flowing of the uncured sealant or the sealant, which becomes a cause of forming a void. Thus, with respect to the mounting by pressing when a semiconductor chip is mounted by pressing against the substrate having a fine unevenness, a flip chip mounting method which does not cause void formation is desired like in the mounting by pressing a semiconductor chip having a bump against the substrate.
Accordingly, a task of the present invention is to provide a flip chip mounting method which does not cause void formation between a semiconductor chip and a substrate.
The present inventors have made extensive and intensive studies with a view toward solving the above-mentioned problems. As a result, they have found that the cause of the void formation resides in the presence of moisture in the substrate, the way of feeding a sealant, and the convection in the underfill caused by the difference in temperature between the semiconductor chip and the substrate, and the task can be achieved by removing the cause, and thus the present invention has been completed.